Presently there are basically two types of herbicides in use. The first type of herbicide is generally applied to the total area of cultivation, and selectively discriminates between the desirable crops and the undesirable weeds, destroying only the latter. In contrast, the second type of herbicide destroys any growing plant to which it is applied. The second type generally tends to be more effective but unique problems are encountered in that application to desirable crops will have the effect of destroying them also. Because various weeds such as quackgrass and the like frequently extend above the upper reaches of the desirable crop, it is advantageous to devise a method by which this second type of herbicide can be applied to the weeds which extend above the desirable crop.
Because the height of both the crops and the weeds vary within any particular acreage, it is advantageous to selectively and continuously control the height at which the herbicide is applied. Similarly, the density of weeds can vary from one acreage to another. In those areas of higher density, it becomes necessary to apply greater volumes of herbicide to the weeds. However, application of great amounts of a nonselective herbicide to weeds creates a serious problem of excess herbicide dripping to the ground. If the herbicide drips too much, an excessive herbicide residue is left behind which kills the desirable crop.